In analyses at very low flow rates, capillary tube flow cells have been frequently used because of their high detection sensitivity.
For example, capillary tube flow cells are often used when mass spectrometers are used as detectors for the capillary electrophoretic process and liquid chromatograph and, in particular, when the detection of absorbance, fluorescence, etc. is performed.
In a method described in the U.S. Pat. No. 5,057,216, a capillary tube is inserted into an optical passage, the capillary material is softened by heating both ends of the capillary tube, and the two ends are bent and formed in the shape of the letter Z or U, whereby a capillary flow cell is fabricated.
And for this bending, the Japanese Patent Laid-Open No. 2002-267597 describes a bending method which involves threading a capillary tube through a fine tube, supporting this fine tube in a sandwiched manner between top and bottom beds which are obtained by division into top and bottom portions, this apparatus being called a fine tube fixing holder, heating both ends of the fine tube, and causing the capillary tube outside the beds to droop down under its own weight, whereby bending is performed.
In a case where a flow cell in which a capillary tube is used is adopted and light is caused to become incident in the same direction as the direction of a flow passage of the capillary tube, it is possible to ensure an optical passage length of several millimeters, and this enables detection sensitivity to be improved.
In conventional methods of fabricating a flow cell in which a capillary tube is used, it is necessary to exfoliate a polyimide coating of a portion which transmits light by use of a gas burner and the like and after that, the capillary tube is bent and formed in the shape of the letter U or Z.
However, because of the minimization of flow rate these capillary tubes have become increasingly small and clogging has become apt to occur. At the same time, because of the diversification of samples, it is necessary to change the diameter of capillary tubes and hence the necessity of replacement of capillary tubes has increased. However, the replacement work is very difficult and requires specialized knowledge and experience and in addition, it takes time to replace capillary tubes. Therefore, general users cannot easily replace capillary tubes. In particular, it takes time to adjust an optical system of a capillary tube, thereby making analysis operations difficult.
Furthermore, in bending a capillary tube, it is necessary to provide a slit in the portion of a capillary tube from which a polyimide coating has been exfoliated, thereby to cut off light other than the light in the optical axis (stray light). However, if the slit diameter is too small on this occasion, detection noise increases because the quantity of transmitted light of the capillary is small.
Furthermore, when transmitted light is supplied and, in particular, when incoming light is weak, an increase in the noise of a detector becomes a serious problem in performance. In addition, because very low flow rates are handled in liquid chromatographic analyses in which a capillary tube is used, flow velocities become low. Therefore, it takes time to perform analyses if the piping is long, thereby bringing about changes due to temperature, quantity of light, etc. and there is a possibility that accurate detection may be impaired.
For this reason, simple replace means of capillary tubes is needed.